In general, the thickness of a dielectric film plays an important role in deciding the coupling ratio of flash memory. However, as devices are highly integrated, there is a tendency that the thickness of a dielectric film gradually decreases. Further, it becomes more difficult to form an isolation layer by fully gap filling a trench without voids using a High Density Plasma (HDP) oxide film, which was used conventionally, because the Critical Dimension (CD) of the trench decreases.
A method of fully gap filling a trench using spin on dielectric (SOD) materials, for example, polysilazane (PSZ) is employed. The PSZ material has a property in which it flows like water due to a low viscosity, and therefore is capable of fully gap fill the trench. However, the PSZ material contains high amounts of impurities and moisture. If the PSZ material is formed adjacent to a tunnel insulating film, the tunnel insulating film degrades. Due to this, a HDP oxide film used as a liner form layer is formed on a surface of the trench in order to prevent the tunnel insulating film from being contaminated due to impurities contained within the PSZ material.
On the other hand, if the HDP oxide film is formed under the PSZ film, the HDP oxide film formed on the sidewalls of a conductive film may not be properly removed. A portion of the HDP oxide film may remain after an etching process for securing the Effective Field oxide Height (EFH) of an isolation layer because the HDP oxide film has a lower etch rate than the PSZ material. The contact area of a floating gate and a dielectric film is thus decreased by the HDP oxide film remaining on the sidewalls of the conductive film in a subsequent deposition process of a dielectric film, so that the coupling ratio reduces. The reduced coupling ratio leads to decreased program speed.
Meanwhile, if a wet etch process is performed additionally in order to remove the HDP oxide film remaining on the sidewalls of the conductive film, the EFH of the isolation layer is further lowered and the tunnel insulating film is attacked accordingly. Consequently, the device is degraded and reliability of the device is lowered.